Isolator for high power laser system

ABSTRACT

A high power laser system having a radiation source and an amplifier with an isolator located in such a position as to reflect radiation from said source to said amplifier. This isolator is made up of an isolator element in intimate contact with a reflector-heat sink and surrounded by a magnetic field. The arrangement of the isolator enables the radiation emanating from the source to be twice passed through the isolator element while affording maximum cooling of the element during operation of the system.

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United sm! Dec. 25, 1973 Schlossberg -V-a-t ISOLATOR FOR HIGH POWERLASER SYSTEM [75] Inventor: Howard Schlossberg, Lexington,

Mass.

[73] Assignee: The United States of America as represented by theSecretary of the Air Force,'Washington, DC.

[22] Filed: Feb. 5, 1973 [21] Appl. No.: 329,510

Related US. Application Data [62] Division of Ser. No. 155,567, June 22,1971,

abandoned.

[52] US. Cl. 331/945 [51] lnt. C1. H01s 3/02 [58] Field of Search...'.331/945 [56] References Cited UNlTED STATES PATENTS 3,456,210 7/1969Statz et al. 33l/94.5 3,523,718 8/1970 Crow 331/945 OSClLLRTOR HMQMfLiFIER 2/1972 Buczek et al. 331/945 9/1972 .lanney 331/945 OTHERPUBLICATIONS Young et al., Traveling-Wave Ruby Laser With a PassiveOptical lsolator, J. Appl. Phys., Vol. 36, No. 10, pg. 3351.

Primary Examiner-William L. Sikes Attorney-Harry A. Herbert, Jr. et a1.

[57] ABSTRACT 5 Claims, 3 Drawing Figures.

ISOLATOR FOR HIGH POWER LASER SYSTEM BACKGROUND OF THE INVENTION Thisinvention relates generally to lasers, and more particularly to animprovement in the isolator element utilized in high power lasers.

The development of the laser has created a new area of technology whichfinds application in many systems already in existence today. Forexample, lasers can be found in the area of optical communications,holography, medicine, cutting, calculating and in radar.

The utilization of the laser in such areas is in many instancesdependent upon the amplification of the existing laser radiation. Inorder to accomplish such an increase in laser power it is necessary foran isolator to be situated between the laser or oscillator producing theradiation and the amplifier.

Lasers of the past utilized an isolator made up of an isolator elementwithin a strong magnetic field adjacent a polarizer set at 45. Theseisolators permitted the transmission of the laser beam in one directionand in so doing rotated the polarization of the beam, thereby allowingthe polarizer to block its transmission in the return direction. Inorder to prevent destruction of the isolator by the reflected amplifiedradiation or by the radiation from the oscillator, in the past theseisolators were cooled at their edges by any conventional cooling system.

In certain areas, such as in optical communication or optical radar, itis necessary to greatly amplify the ini- 'tial radiation power producedby the laser. Heretofore, such an amplification proved to be highlyimpractical since the existing isolators for high power lasers werecooled at their edges. Because the heat deposited by the radiation inisolators of the past was carried to the edge of the element, theradiation power and aperture of the element were limited by theallowable temperature rise in the element center or by stress due tothermal gradients. Therefore any increased radiation returned by theamplifier was of such strength as to destroy'vthe isolator element andfurther, damage the oscillator or laser from which such power emanated.

In areas of use, wherein a large power output laser is required, theexpense involved in the past made the cost of the project or systemprohibitive. Heretofore, no isolator has been economically developedwhich could accommodate the large heat transfer involved inamplification of laser power.

SUMMARY OF THE INVENTION able aperture of the isolator is unlimited withthe stress small, uniform and compressional so that material damage dueto them is unlikely. It is further possible to liquid nitrogen cool-thereflector element if desired to provide a larger heat dissipation.

It is preferable to use an Indium antimonide crystal as the isolatorelement located within a magnetic field and being in intimate contactwith a well polished copper mirror acting as the reflector and heatsink. This crystal may be held to the copper mirror with a thermallyconducting paste applied near its edges or with spring clips attached tothe copper. In the instant invention the radiation emanating from theoscillator or laser is allowed to make two passes through thelndiumantimonide crystal at a very small angle to its axis and is reflected bythe copper mirror to an amplifier. Any reflected radiation from theamplifier in the direction of the oscillator will be blocked by apolarizer situated in front of the oscillator.

Because of the reflecting surface and heat sink of the copper mirroradjacent the crystal, any heating of the crystal which takes place dueto the increase in power will be conducted in a uniform manner throughthe thin dimension of the crystal. Since the temperature and stressdistribution are uniform, the distortion of the input radiation beam isfar lower than for the edge cooled isolator of the past. This principlecan be readily extended to more than one reflection and amplification ifnecessary, something almost totally impossible with the isolators of thepast.

It is therefore an object of this invention to provide an isolatorcapable of handling large amounts of laser radiation passingtherethrough.

It is another object of this invention to provide an isolator whichuniformly conducts heat along the thin dimension of the isolatorelement.

It is a further object of this invention to provide an isolator in whichthe incident radiation from a source passes twice through the isolatorelement before reaching the amplifier.

It is still another object of this invention to provide an isolatorwhich is economical to produce and which utilizes conventional,currently available components that lend themselves to standard massproducing manufacturing techniques.

For a better understanding of the present invention together with otherand further objects thereof, reference is made to the followingdescription taken in connection with the accompanying drawing and itsscope .will be pointed out in the appended claims.

DESCRIPTION OF THE DRAWING FIG. 1 is a schematic drawing of the priorart isolator used in combination with an oscillator and amplifier;

FIG. 2 is a schematic drawing of the isolator of this invention used incombination with an oscillator and amplifier; and

FIG. 3 is a schematic drawing of a plurality of isolators of thisinvention used in combination with an oscillator and amplifier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Reference is now madeto FIG. I of the drawing which shows in schematic fashion the isolator10 of the prior art. This isolator is in the form of an isolator element12 located within a strong magnetic field 14. Any conventional coolingsystem 16 is utilized to dissipate the heat within the element 12. Theisolator 10 is positioned between the laser or oscillator 18 andamplifier 20 with a polarizer 22 adjacent oscillator 18. In operationthe radiation or laser beam 24 emanating from oscillator 18 passesthrough polarizer 22 and isolator 10 before entering the amplifier 20.The isolator 10, be-

cause of the strong magnetic field associated therewith, rotates" beam24 during passage therethrough. Because of thisbeam rotation in acontinuous direction upon passing through a magnetic field (FaradayEffect) any return or reflected amplified radiation 26 from amplifier 20will be blocked by polarizer 22 from reaching oscillator 18.

As can be seen from FIG. 1, the heat 27 generated by the amplifiedradiation 26 passing through isolator element 12 is carried to the edgesof element 12 where it is cooled by system 16. Such a cooling system 16,however, it totally deficient when it is necessary to cool the largeamounts of heat produced by greatly amplified radiation. The excess heatwithin the isolator element 12 not only destroys the element itself butthereafter also allows the return radiation to damage oscillator 18. Itcan be clearly seen that the isolator system of the past lacked theproper cooling capacity necessary to handle the large amounts ofradiation power of laser communication and radar systems.

The instant invention is best shown in FIG. 2 of the drawing. In theisolator 28 of this invention the isolator element 30 is made of anysuitable material such as an indium antimonide crystal 0.5 mm thick(e.g. l cm in diameter) with 2X10 carrier electrons/cm placed within amagnetic field 32 of approximately 2500 gauss. The surfaces of theisolator element 30 are optically polished and the element 30 is placedin intimate contact with a well polished reflector-heat sink 34preferably of copper. This reflector-heat sink 34 is cooled by anysuitable substance 36 such as water or liquid nitrogen. The isolatorelement 30 is held in place on copper reflector-heat sink 34 with anythermally conducting paste applied near its surface or with spring clips(not shown) attached to the reflector-heat sink 34. If necessary forintimate contact a thin gold layer may be deposited on the isolatorsurface.

With the isolator 28 of the instant invention the laser source oroscillator 38 and the amplifier 40 are placed on the same side of theisolator 28. Because of this relationship the incident radiation or beam42 makes two passes through the isolator element 30 at a very smallangle, having been reflected by the copper refiectorheat sink 34. Bymaking two passes through the isolator element 30 the correspondingmagnetic field 32 necessary for beam rotation may be reduced by a factorof two. (It is also possible that the magnetic field remain constant eg500 gauss and the isolator element 30 be of half the thickness used.)

As with the isolator 10 of the prior art a polarizer 44 is locatedadjacent the oscillator 38 to prevent any reflected radiation 46 fromamplifier 40 from reaching oscillator 38.

The isolator '28 in combination with the oscillator 38 and amplifier 40as set forth in FIG. 2 greatly increases the laser power handlingcapacity over systems of the past which are cooled at the edge of theisolator element since, due to the high diameter to thickness ratio heatwill be uniformly conducted through the 0.5 mm thickness (thindimension) into the copper reflectorheat sink 34 as shown by arrows 48.Because of the arrangement of this invention the aperture of theisolator element is unlimited in size and thermal stress is uniform andalong the thin dimension. Since the temperature and stress distributionsare uniform, the distortion of the input radiation beam is far lowerthan for the edge cooled isolators 10 of the prior art. It is possibleto handle radiation flux in excess of I kilowatt/cm with the instantinvention.

Still referring to FIG. 2, in operation the radiation beam 42 emanatingfrom oscillator 38; l passes through polarizer 44; (2) passes throughisolator element 30 where it is rotated;" (3) reflects of copper mirror34; (4) passes back through isolator element 30 where it is stillfurther rotated;" and (5) passes onto amplifier where it is amplifiedfor further use. Any reflected radiation 46 from amplifier 40 returnsthrough isolator element 30 where the heat accumulated therein conductsthrough the thin dimension of the isolator element 30 to copper heatsink 34. The reflected radiation 46 is blocked by polarizer 44.

Not only does this invention allow for great amplification of radiationpower without damage to the isolator 30 and oscillator 38, but thisinvention also allows for a reduction of the magnetic field 32 necessaryto rotate the radiation beam due to the double pass of incidentradiation through the isolator element 30.-

Reference is now made to FIG. 3 of the drawing wherein like numeralswill designate identical elements of FIG. 2. As shown in FIG. 3, theisolator system of this invention can be readily extended to more thanone reflection if necessary, a, process heretofore virtually impossible.With the use of two or more isolators 28, the incident radiation poweremenating from oscillator 38 can be even more greatly increased. Forgiven magnetic fields this procedure allows the isolator elements 30 tobe of one-fourth the thickness otherwise necessary, thereby furtherreducing the heat load on each isolator element 30 for a given heatflux;

Although this invention has been described with reference to aparticular embodiment, it will be understood to those skilled in the artthat the isolator of this invention could also be used as part of acirculator at the output of a radar (in order to use the same telescopefor transmitting as well as receiving) or between a preamplifier and anamplifier all within the spirit and scope of the appended claims.

I claim:

l. A high power laser system comprising a laser source for producing ahigh power laser beam, an isolator located adjacent said laser source inoptical alignment with said laser beam, said isolator comprising a rellecto.r.=heat sink an element which exhibits a Faraday Effect fixedlysecured to said reflector-heat sink, and means surrounding said elementfor causing said element to produce a beam rotation therein, a polarizerlocated between said laser source and said isolator and an amplifierlocated on the same side of said isolator as said laser source wherebysaid laser beam passes through said polarizer and through said elementwherein it is rotated, reflects off said reflector-heat sink, passesback through said element wherein it is still further rotated and passesonto said amplifier, any reflected laser beam from said amplifier beingfurther rotated by said isolator before passing onto said polarizerwhere it is prevented from reaching said laser source.

2. A high power laser system as defined in claim 1 wherein said isolatorelement is held in contact with said reflector-heat sink by a thennallyconducting paste.

3. A high power laser system as defined in claim 2 wherein saidreflector-heat sink is made of copper.

4. A high power laser system as defined in claim 1 wherein a secondisolator is located adjacent said first sink, an element which exhibitsa Faraday Effect fixedly secured thereto and means surrounding saidelement for causing said element to produce a beam rotation therein.

1. A high power laser system comprising a laser source for producing ahigh power laser beam, an isolator located adjacent said laser source inoptical alignment with said laser beam, said isolator comprising areflector-heat sink, an element which exhibits a Faraday Effect fixedlysecured to said reflector-heat sink, and means surrounding said elementfor causing said element to produce a beam rotation therein, a polarizerlocated between said laser source and said isolator and an amplifierlocated on the same side of said isolator as said laser source wherebysaid laser beam passes through said polarizer and through said elementwherein it is rotated, reflects off said reflector-heat sink, passesback through said element wherein it is still further rotated and passesonto said amplifier, any reflected laser beam from said amplifier beingfurther rotated by said isolator before passing onto said polarizerwhere it is prevented from reaching said laser source.
 2. A high powerlaser system as defined in claim 1 wherein said isolator element is heldin contact with said reflector-heat sink by a thermally conductingpaste.
 3. A high power laser system as defined in claim 2 wherein saidreflector-heat sink is made of copper.
 4. A high power laser system asdefined in claim 1 wherein a second isolator is located adjacent saidfirst isolator thereby receiving said reflected laser beam therefrom,said second isolator reflecting said laser beam to said amplifier.
 5. Ahigh power laser system as defined in claim 4 wherein said secondisolator comprises a reflector-heat sink, an element which exhibits aFaraday Effect fixedly secured thereto and means surrounding saidelement for causing said element to produce a beam rotation therein.